Field of the Invention
Embodiments of the present invention generally relate to a casing string for a wellbore.
Description of the Related Art
Wellbores typically include a casing string that structurally supports the walls of the wellbore and isolates the wellbore from the surrounding geological formations. In many instances, an annular gap between the wellbore and the casing is filled with cement. Referring to FIG. 1A, a well 100 includes a wellbore 102 that is formed by a drill bit. When the wellbore 102 reaches a depth at which the walls of the wellbore may collapse (e.g., due to pressures exerted by the surrounding geological formations), a casing string 110 is placed in the wellbore. The casing string 110 is first positioned in the wellbore 102. Then, a cement slurry is pumped through the casing string 110 and out through one or more openings 114 at the bottom of the casing string 110. For example, the casing string 110 may include a shoe that guides the casing string 110 into the wellbore 102. The shoe can include one or more ports through which the cement slurry can pass into the annular gap 120 (in the direction of arrow A). The cement slurry can be pumped so that it travels through the wellbore 102 back toward the surface in the annular gap 120 between the walls 112 of the casing string 110 and the wellbore 102.
In various instances, the cement slurry may not be able to be pumped through the annular gap 120 to the top of the wellbore 102 (or the top of the casing string 110). As an illustration, the cement slurry may only be pumped to a height indicated by dashed line 122 in the annular gap 120. For example, a cement slurry pump may only provide sufficient pressure to pump the cement slurry to the height of the dashed line 122. As another example, pumping the cement slurry to a height above the dashed line 122 may require a hydrostatic and/or applied pressure of the cement slurry that exceeds a fracture pressure of geological structures surrounding the wellbore 102. In such instances, a port 116 can be included in the casing string through which the cement slurry can flow (in the direction of arrow B). As the cement slurry reaches the height of the dashed line 122, a plug can be sent through the casing string 110 that closes off the openings 114 at the bottom of the casing string. The plug also pushes remaining cement slurry out of the casing string 110 and into the annular gap 120. After the plug reaches the bottom of the casing string, pressure within the casing string increases until a rupture disc in the port 116 bursts, enabling cement slurry to flow out of the port in the direction of arrow B. The cement slurry can then fill the annular gap 120 above the dashed line 122. The casing string 110 may include more than one port 116 along its length, and the above-described process of plugging the casing string and bursting a rupture disc can be sequentially repeated to fill the annular gap 120 with cement slurry. Additionally, the casing string 110 may include more than one port 116 at each lengthwise location. By providing multiple ports and rupture discs at each location, redundancy can be provided in case a rupture disc fails to burst.
FIGS. 1B-1D illustrate in greater detail a process for providing cement slurry to the annular gap 120 between the wellbore 102 and the walls 112 of the casing string 110. FIG. 1B shows a first casing string section 112a and a second casing string section 112b that are joined by a body 118. For example, the first casing string section 112a can include external threads that engage internal threads on the body 118. Similarly, the second casing string section 112b can include external threads that engage internal threads on the body 118. The first casing string section 112a and the second casing string section 112b are separated by a gap 130 when they are engaged in the body 118. The body 118 includes a port 132 therethrough. The port 132 includes a rupture disc 116 that temporarily blocks the port 132 and prevents cement slurry from flowing through the port 132 and into the annular gap 120 between the casing string sections 112a, 112b and the wellbore 102. A sealing sleeve 140 is arranged in the first casing string section 112a at a location that is upstream from the gap 130 and the port 132. The sealing sleeve 140 can include one or more resilient members 142 at an upstream end and one or more resilient members 144 at a downstream end. In FIG. 1B, the cement slurry is moving past the port 132 toward the downhole end of the casing string 110, as indicated by arrow A. As discussed above, after the cement slurry has been pumped through the annular gap 120 to a particular height (or when a threshold hydrostatic and/or applied pressure of the cement slurry has been reached), a plug, dart, or the like can be sent through the casing string 110 to block openings through which the cement slurry is passing to reach the annular gap 120. Thereafter, a pressure rise within the casing string 110 causes the rupture disc 116 to burst. FIG. 1C shows the port 132 after the rupture disc has burst. After the rupture disc 116 has burst, cement slurry can flow out through the port 132 as indicated by arrow B.
Within the casing string 110, the cement slurry flows past the walls of the casing sections 112a and 112b proximate to the port. As a result, the walls of the casing sections 112a and 112b proximate to the port may suffer erosion from the flowing cement slurry, as indicated by rounded portions 134 of the walls of the casing sections 112a and 112b. By contrast, a side of the casing string 110 opposite the port 132 may not suffer any erosion because the cement slurry is generally stagnant at that location.
After the cement slurry has been pumped through the port 132, the port 132 can be isolated and sealed by moving the sealing sleeve 140 in the direction of arrow C, as shown in FIG. 1D. For example, a plug or a dart can be sent through the casing section 112a to push the sealing sleeve 140. After the sealing sleeve 140 has been moved, the resilient members 142 at the upstream end of the sealing sleeve and the resilient members 144 at the downstream end of the sealing sleeve press against the walls of the casing string sections 112a and 112b to isolate the port 132 from the interior of the casing string 110. The resilient members 142 and 144 are used because the erosion of the walls of the casing string 110 caused by the cement slurry (indicated by rounded portions 134) can result in an irregular surface finish to the interior of the walls of the casing string 110. The resilient members 142 and 144 conform to such irregular surfaces to provide a seal. However, such resilient members 142 and 144 may lack long-term durability. For example, resilient members made of rubber, plastic, or a polymer may degrade over time and allow oil, gas, and/or a drilling fluid to reach the cement in the annulus 120. The oil, gas, or drilling fluid could weaken the cement in the annulus 120 and possibly compromise the well 100.